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SIADH associated with lung malignancy — SCE Acute Medicine MCQ

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ModerateRespiratory medicineSIADH associated with lung malignancySCE Acute Medicine

A 66-year-old man with fever and productive cough has a sodium of 119 mmol/L. Serum osmolality is low, urine osmolality is 620 mOsm/kg and urine sodium is 58 mmol/L. Chest radiograph shows a right hilar mass with post-obstructive pneumonia. He is euvolaemic on examination. What is the most likely underlying mechanism?

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Correct answer: A — Ectopic antidiuretic hormone secretion causing SIADH

Euvolaemic hypotonic hyponatraemia with concentrated urine and high urine sodium is SIADH. A hilar mass raises concern for small-cell lung cancer, a classic cause of ectopic ADH, and pneumonia can also contribute. Psychogenic polydipsia would suppress ADH and produce dilute urine.

Reference: NICE CKS Hyponatraemia; Society for Endocrinology Guidance